Recently, various electronic devices of a form directly wearable on the human body are being developed. These devices are commonly called wearable electronic devices. Examples of the wearable electronic devices can include a head-mounted display, smart glasses, a smart watch or wristband, a contact lens type device, a ring type device, a shoe type device, a clothes type device, a glove type device, etc., and have various forms being attachable to or detachable from a part of the human body or clothes. The wearable electronic devices are directly wearable on the human body to enhance their portability and user's accessibility.
One example of the wearable electronic devices is a device mountable on a user's head. Such a device can be, for example, called a Head-Mounted Display or Head-Mounted Device (HMD).